Differential Expression of the Apolipoprotein AI Gene in Spotnape Ponyfish (Nuchequula nuchalis) Inhabiting Different Salinity Ranges at the Top of the Estuary and in the Deep-Bay Area of Gwangyang Bay, South Korea

Spotnape ponyfish (Nuchequula nuchalis) is a dominant species that is broadly distributed from estuarine to deep-bay areas, reflecting a euryhaline habitat. Apolipoprotein AI (ApoAI) is a main component of plasma lipoproteins and has crucial roles in lipid metabolism and the defense immune system. In this study, we characterized the N. nuchalis ApoAI gene and analyzed the expression of the ApoAI transcript in N. nuchalis collected at various sites in the estuary and the deep-bay area which have different salinities. Owing to the fish’s mobility, we conducted stable isotope analyses to confirm the habitat characteristics of N. nuchalis. Carbon and nitrogen isotope ratios (δ(13)C and δ(15)N) from N. nuchalis indicated different feeding sources and trophic levels in the estuarine and deep-bay habitats. The characterized N. nuchalis ApoAI displayed residual repeats that formed a pair of alpha helices, indicating that the protein belongs to the apolipoprotein family. In the phylogenetic analysis, there was no sister group of N. nuchalis ApoAI among the large clades of fish species. The transcriptional expression level of ApoAI was higher in N. nuchalis inhabiting the deep-bay area with a high salinity (over 31 psu) than in N. nuchalis inhabiting the top of the estuary with a low salinity (6~15 psu). In addition, the expression patterns of N. nuchalis ApoAI were positively correlated with environmental factors (transparency, pH, TC, and TIC) in the high salinity area. These results suggest that ApoAI gene expression can reflect habitat characteristics of N. nuchalis which traverse broad salinity ranges and is associated with functional roles of osmoregulation and lipid metabolism for fish growth and development.